From: Christian Brauner <christian.brauner@ubuntu.com>
To: Florian Weimer <fw@deneb.enyo.de>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
Oleg Nesterov <oleg@redhat.com>,
Christian Brauner <christian@brauner.io>,
Jann Horn <jannh@google.com>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Daniel Colascione <dancol@google.com>,
Joel Fernandes <joel@joelfernandes.org>,
linux-man <linux-man@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>,
lkml <linux-kernel@vger.kernel.org>
Subject: Re: For review: pidfd_send_signal(2) manual page
Date: Mon, 23 Sep 2019 16:23:27 +0200 [thread overview]
Message-ID: <20190923142325.jowzbnwjw7g7si7j@wittgenstein> (raw)
In-Reply-To: <87pnjr9rth.fsf@mid.deneb.enyo.de>
On Mon, Sep 23, 2019 at 01:26:34PM +0200, Florian Weimer wrote:
> * Michael Kerrisk:
>
> > SYNOPSIS
> > int pidfd_send_signal(int pidfd, int sig, siginfo_t info,
> > unsigned int flags);
>
> This probably should reference a header for siginfo_t.
Agreed.
>
> > ESRCH The target process does not exist.
>
> If the descriptor is valid, does this mean the process has been waited
> for? Maybe this can be made more explicit.
If by valid you mean "refers to a process/thread-group leader" aka is a
pidfd then yes: Getting ESRCH means that the process has exited and has
already been waited upon.
If it had only exited but not waited upon aka is a zombie, then sending
a signal will just work because that's currently how sending signals to
zombies works, i.e. if you only send a signal and don't do any
additional checks you won't notice a difference between a process being
alive and a process being a zombie. The userspace visible behavior in
terms of signaling them is identical.
>
> > The pidfd_send_signal() system call allows the avoidance of race
> > conditions that occur when using traditional interfaces (such as
> > kill(2)) to signal a process. The problem is that the traditional
> > interfaces specify the target process via a process ID (PID), with
> > the result that the sender may accidentally send a signal to the
> > wrong process if the originally intended target process has termi‐
> > nated and its PID has been recycled for another process. By con‐
> > trast, a PID file descriptor is a stable reference to a specific
> > process; if that process terminates, then the file descriptor
> > ceases to be valid and the caller of pidfd_send_signal() is
> > informed of this fact via an ESRCH error.
>
> It would be nice to explain somewhere how you can avoid the race using
> a PID descriptor. Is there anything else besides CLONE_PIDFD?
If you're the parent of the process you can do this without CLONE_PIDFD:
pid = fork();
pidfd = pidfd_open();
ret = pidfd_send_signal(pidfd, 0, NULL, 0);
if (ret < 0 && errno == ESRCH)
/* pidfd refers to another, recycled process */
>
> > static
> > int pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
> > unsigned int flags)
> > {
> > return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
> > }
>
> Please use a different function name. Thanks.
next prev parent reply other threads:[~2019-09-23 14:23 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-23 9:12 For review: pidfd_send_signal(2) manual page Michael Kerrisk (man-pages)
2019-09-23 11:26 ` Florian Weimer
2019-09-23 14:23 ` Christian Brauner [this message]
2019-09-24 19:44 ` Michael Kerrisk (man-pages)
2019-09-24 19:57 ` Christian Brauner
2019-09-24 20:07 ` Christian Brauner
2019-09-24 21:00 ` Michael Kerrisk (man-pages)
2019-09-24 21:08 ` Daniel Colascione
2019-09-25 13:46 ` Michael Kerrisk (man-pages)
2019-09-24 21:53 ` Christian Brauner
2019-09-25 13:46 ` Michael Kerrisk (man-pages)
2019-09-25 13:51 ` Florian Weimer
2019-09-25 14:02 ` Michael Kerrisk (man-pages)
2019-09-25 13:53 ` Christian Brauner
2019-09-25 14:29 ` Michael Kerrisk (man-pages)
2019-09-24 19:43 ` Michael Kerrisk (man-pages)
2019-09-25 1:48 ` Jann Horn
2019-09-23 11:31 ` Daniel Colascione
2019-09-24 19:42 ` Michael Kerrisk (man-pages)
2019-09-23 14:29 ` Christian Brauner
2019-09-23 20:27 ` Michael Kerrisk (man-pages)
2019-09-23 21:27 ` Eric W. Biederman
2019-09-24 19:10 ` Michael Kerrisk (man-pages)
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